Generally, a fuel cell is an electric power generation system that converts chemical energy directly into electric energy through an electrochemical reaction fueled by hydrogen contained in a hydrocarbon-based organic fuel such as methanol, ethanol or natural gas. Since organic fuel has a high specific energy, a fuel cell system using organic fuel is extremely attractive in the areas of both setup and portability. For example, the specific energy of methanol is 6,232 Wh/kg.
Fuel cells are often categorized as Phosphoric Acid Fuel Cells (PAFC) that operate at around 150 to 200° C., Molten Carbonate Fuel Cells (MCFC) that operate at a high temperature of 600 to 700° C., Solid Oxide Fuel Cells (SOFC) that operate at a high temperature over 1,000° C., and Proton Exchange Membrane Fuel Cells (PEMFC) and Alkaline Fuel Cells(AFC) that operate in a range between room temperature and a temperature not higher than 100° C. The fuel cells operate based on the same fundamental principles. However, the kind of fuel used, the operating temperature, the catalyst and the electrolyte may differ.
Among fuel cells, the PEMFC, which has been developed recently, has excellent output characteristics, fast starting and response characteristics and a low operating temperature compared to other types of fuel cells. It also has a wide application range and can be used as a distributed power source for houses and public buildings or as a small power source for electronic devices. In addition, the PEMFC is useful as a mobile power source, and for example, may be used in a car by using hydrogen obtained by reforming methanol, ethanol or natural gas as fuel.
The basic structure of a PEMFC system includes a fuel cell body called a stack, a fuel tank, a fuel pump and a reformer for generating hydrogen gas by reforming the fuel. Therefore, the PEMFC generates electric energy by supplying the fuel stored in the fuel tank to the reformer, generating hydrogen gas through reformation in the reformer, and causing the hydrogen gas to react with oxidant electro-chemically in the stack.
Fuel cells can also use a Direct Oxidation Fuel Cell (DOFC) scheme that can supply liquid-phase methanol fuel to the stack directly. The fuel cell of the DOFC scheme does not require the reformer, which is different from the PEMFC.
In the above fuel cell system, the stack that generates electricity substantially includes several to scores of unit cells stacked in multi-layers and each unit cell is formed of a membrane-electrode assembly (MEA) and a bipolar plate. The membrane-electrode assembly has an anode and a cathode attached to each other with an electrolyte membrane between them. The bipolar plate acts as a path for supplying hydrogen gas and oxidant, which are required for the reaction of the fuel cell. In addition, the bipolar plate connects the anode and cathode of the membrane-electrode assembly serially. Due to the bipolar plate, hydrogen gas is supplied to the anode whereas oxidant is supplied to the cathode. During the process, the hydrogen gas goes through an electrochemical oxidation reaction in the anode and the oxidant goes through an electrochemical reduction reaction in the cathode. Due to the transfer of electrons during the reactions, electricity is obtained along with heat and water.
The aforementioned reformer eliminates harmful materials such as carbon monoxide which deactivates the fuel cell and shortens the life of the fuel cell, as well as converting hydrogen into hydrogen gas which is needed to generate electricity in the stack by reforming hydrogen-containing fuel with water.
In the case of a fuel cell for mobile applications requiring a reformer, the size of the reformer is so small that the width and depth of a flow channel are between scores of micrometers and scores of millimeters. In this reformer, however, a catalyst layer cannot be formed precisely in a conventional slurry injection method or direct coating method. Moreover, since the specific surface area is small, the reforming effect may not be sufficient.